This invention relates generally to proximity detectors and more particularly to a detector for determining the lateral position of a conductive band moving along a path within the magnetic field of an inductor.
In a belt or band type of high speed printer, a metal band having on its surface embossed or etched characters is driven continuously about a driving pulley and driven idler. Along one linear portion of the path on one side of the band is a platen and adjacent the opposite side of the band at that location are a reversible, transversely moving ribbon, a recording medium of paper and a plurality of selectively energizable hammers that impact the paper and ribbon against type characters that are in turn pushed against the platen. Correct positioning of the band is important to good print registration.
When several of the hammers are energized concurrently or nearly so there is a momentary urging by the movement of the ribbon tending to slightly displace the band transversely of its path. Since the band moves at high speed and several impacts are required to fill the line of print, the print registration varies considerably resulting in poor appearance. Band wear also contributes to the band tracking problem.
The bands can be interchanged in this type of printer to permit replacement or the use of different type fonts. Although the bands are manufactured to close tolerances, the band dimensions, mounting stresses and repeated impacts are not exactly the same so that different bands do not track in exactly the same path.
Manual adjustments are typically provided to enable band correction by the user. Although such corrections can be done in the latter instance of changing bands, it is frequently time consuming. Accordingly, automatic adjustment is highly desirable and such adjustments should be made to correct misalignments on the order of 0.1 to 0.2 millimeters of deviation from the band path. Optical and Hall or magnetoresistive devices have been tried for sensing band position, but these require expensive components or material and are unstable due to dirt, magnetic characteristics of the band, or dimensional change.
One known technique for sensing the position of a character band is that shown in an article by E. F. Helinski entitled "Belt Edge Detector" in the IBM Technical Disclosure Bulletin, Vol. 23, No. 8, January, 1981, pages 3641-3. With this technique the band moves within the gap of a magnetic core having a pair of windings. One winding, serving as a transmitting coil is energized with a high frequency signal and the other winding, serving as the receiving coil, is tapped to permit picking off the induced voltage. As the band position moves transversely of its normal path farther into or out of the gap of the transducer, the sensed voltage correspondingly decreases or increases. The magnitude of the induced signal, apparently influenced by eddy currents induced in the band, is compared with calibrated threshold values to indicate the amount of displacement from the band's desired position. This method has the disadvantage of being useful and reliable only for changes in position of 0.6 to 0.8 mm. These changes are greater than can be tolerated for acceptable printing registration.
An alternative detection scheme, shown in U.S. Pat. No. 3,735,229, is similar but, again, does not have the required sensitivity. In this servo system arrangement, an oscillator periodically energizes a resonant tank circuit of inductor and capacitor. A portion of a movable conductive metal element lies within the inductor and has eddy currents induced therein. The eddy currents effect a reduction of the energy available to the tank circuit thus affecting the rate of decay of the tank circuit oscillations. Constant integration of the voltage across the tank circuit provides an indication of the element's position. The integrated voltage is compared with a threshold and an error signal energizes the servo to change element position. This technique, while insensitive to the accumulation of debris, is inadequate to sense the small changes required for controlling a printer band location. By integrating the oscillating tank circuit voltage there is permitted a relatively large variation that is also sensitive to voltage supply and temperature changes. Further, the integration requires the currents of several cycles of operation to indicate reliably the element's position.